Provided by: qemu-utils_4.2-3ubuntu6.30_amd64 
NAME
qemu-img - QEMU disk image utility
SYNOPSIS
qemu-img [standard options] command [command options]
DESCRIPTION
qemu-img allows you to create, convert and modify images offline. It can handle all image formats
supported by QEMU.
Warning: Never use qemu-img to modify images in use by a running virtual machine or any other process;
this may destroy the image. Also, be aware that querying an image that is being modified by another
process may encounter inconsistent state.
OPTIONS
Standard options:
-h, --help
Display this help and exit
-V, --version
Display version information and exit
-T, --trace [[enable=]pattern][,events=file][,file=file]
Specify tracing options.
[enable=]pattern
Immediately enable events matching pattern (either event name or a globbing pattern). This
option is only available if QEMU has been compiled with the simple, log or ftrace tracing
backend. To specify multiple events or patterns, specify the -trace option multiple times.
Use "-trace help" to print a list of names of trace points.
events=file
Immediately enable events listed in file. The file must contain one event name (as listed in the
trace-events-all file) per line; globbing patterns are accepted too. This option is only
available if QEMU has been compiled with the simple, log or ftrace tracing backend.
file=file
Log output traces to file. This option is only available if QEMU has been compiled with the
simple tracing backend.
The following commands are supported:
amend [--object objectdef] [--image-opts] [-p] [-q] [-f fmt] [-t cache] -o options filename
bench [-c count] [-d depth] [-f fmt] [--flush-interval=flush_interval] [-n] [--no-drain] [-o offset]
[--pattern=pattern] [-q] [-s buffer_size] [-S step_size] [-t cache] [-w] [-U] filename
check [--object objectdef] [--image-opts] [-q] [-f fmt] [--output=ofmt] [-r [leaks | all]] [-T src_cache]
[-U] filename
commit [--object objectdef] [--image-opts] [-q] [-f fmt] [-t cache] [-b base] [-d] [-p] filename
compare [--object objectdef] [--image-opts] [-f fmt] [-F fmt] [-T src_cache] [-p] [-q] [-s] [-U]
filename1 filename2
convert [--object objectdef] [--image-opts] [--target-image-opts] [-U] [-C] [-c] [-p] [-q] [-n] [-f fmt]
[-t cache] [-T src_cache] [-O output_fmt] [-B backing_file] [-o options] [-l snapshot_param] [-S
sparse_size] [-m num_coroutines] [-W] [--salvage] filename [filename2 [...]] output_filename
create [--object objectdef] [-q] [-f fmt] [-b backing_file] [-F backing_fmt] [-u] [-o options] filename
[size]
dd [--image-opts] [-U] [-f fmt] [-O output_fmt] [bs=block_size] [count=blocks] [skip=blocks] if=input
of=output
info [--object objectdef] [--image-opts] [-f fmt] [--output=ofmt] [--backing-chain] [-U] filename
map [--object objectdef] [--image-opts] [-f fmt] [--output=ofmt] [-U] filename
measure [--output=ofmt] [-O output_fmt] [-o options] [--size N | [--object objectdef] [--image-opts] [-f
fmt] [-l snapshot_param] filename]
snapshot [--object objectdef] [--image-opts] [-U] [-q] [-l | -a snapshot | -c snapshot | -d snapshot]
filename
rebase [--object objectdef] [--image-opts] [-U] [-q] [-f fmt] [-t cache] [-T src_cache] [-p] [-u] -b
backing_file [-F backing_fmt] filename
resize [--object objectdef] [--image-opts] [-f fmt] [--preallocation=prealloc] [-q] [--shrink] filename
[+ | -]size
Command parameters:
filename
is a disk image filename
fmt is the disk image format. It is guessed automatically in most cases. See below for a description of
the supported disk formats.
size
is the disk image size in bytes. Optional suffixes "k" or "K" (kilobyte, 1024) "M" (megabyte, 1024k)
and "G" (gigabyte, 1024M) and T (terabyte, 1024G) are supported. "b" is ignored.
output_filename
is the destination disk image filename
output_fmt
is the destination format
options
is a comma separated list of format specific options in a name=value format. Use "-o ?" for an
overview of the options supported by the used format or see the format descriptions below for
details.
snapshot_param
is param used for internal snapshot, format is 'snapshot.id=[ID],snapshot.name=[NAME]' or
'[ID_OR_NAME]'
--object objectdef
is a QEMU user creatable object definition. See the qemu(1) manual page for a description of the
object properties. The most common object type is a "secret", which is used to supply passwords
and/or encryption keys.
--image-opts
Indicates that the source filename parameter is to be interpreted as a full option string, not a
plain filename. This parameter is mutually exclusive with the -f parameter.
--target-image-opts
Indicates that the output_filename parameter(s) are to be interpreted as a full option string, not a
plain filename. This parameter is mutually exclusive with the -O parameters. It is currently required
to also use the -n parameter to skip image creation. This restriction may be relaxed in a future
release.
--force-share (-U)
If specified, "qemu-img" will open the image in shared mode, allowing other QEMU processes to open it
in write mode. For example, this can be used to get the image information (with 'info' subcommand)
when the image is used by a running guest. Note that this could produce inconsistent results because
of concurrent metadata changes, etc. This option is only allowed when opening images in read-only
mode.
--backing-chain
will enumerate information about backing files in a disk image chain. Refer below for further
description.
-c indicates that target image must be compressed (qcow format only)
-h with or without a command shows help and lists the supported formats
-p display progress bar (compare, convert and rebase commands only). If the -p option is not used for a
command that supports it, the progress is reported when the process receives a "SIGUSR1" or "SIGINFO"
signal.
-q Quiet mode - do not print any output (except errors). There's no progress bar in case both -q and -p
options are used.
-S size
indicates the consecutive number of bytes that must contain only zeros for qemu-img to create a
sparse image during conversion. This value is rounded down to the nearest 512 bytes. You may use the
common size suffixes like "k" for kilobytes.
-t cache
specifies the cache mode that should be used with the (destination) file. See the documentation of
the emulator's "-drive cache=..." option for allowed values.
-T src_cache
specifies the cache mode that should be used with the source file(s). See the documentation of the
emulator's "-drive cache=..." option for allowed values.
Parameters to snapshot subcommand:
snapshot
is the name of the snapshot to create, apply or delete
-a applies a snapshot (revert disk to saved state)
-c creates a snapshot
-d deletes a snapshot
-l lists all snapshots in the given image
Parameters to compare subcommand:
-f First image format
-F Second image format
-s Strict mode - fail on different image size or sector allocation
Parameters to convert subcommand:
-n Skip the creation of the target volume
-m Number of parallel coroutines for the convert process
-W Allow out-of-order writes to the destination. This option improves performance, but is only
recommended for preallocated devices like host devices or other raw block devices.
-C Try to use copy offloading to move data from source image to target. This may improve performance if
the data is remote, such as with NFS or iSCSI backends, but will not automatically sparsify zero
sectors, and may result in a fully allocated target image depending on the host support for getting
allocation information.
--salvage
Try to ignore I/O errors when reading. Unless in quiet mode ("-q"), errors will still be printed.
Areas that cannot be read from the source will be treated as containing only zeroes.
Parameters to dd subcommand:
bs=block_size
defines the block size
count=blocks
sets the number of input blocks to copy
if=input
sets the input file
of=output
sets the output file
skip=blocks
sets the number of input blocks to skip
Command description:
amend [--object objectdef] [--image-opts] [-p] [-q] [-f fmt] [-t cache] -o options filename
Amends the image format specific options for the image file filename. Not all file formats support
this operation.
bench [-c count] [-d depth] [-f fmt] [--flush-interval=flush_interval] [-n] [--no-drain] [-o offset]
[--pattern=pattern] [-q] [-s buffer_size] [-S step_size] [-t cache] [-w] [-U] filename
Run a simple sequential I/O benchmark on the specified image. If "-w" is specified, a write test is
performed, otherwise a read test is performed.
A total number of count I/O requests is performed, each buffer_size bytes in size, and with depth
requests in parallel. The first request starts at the position given by offset, each following
request increases the current position by step_size. If step_size is not given, buffer_size is used
for its value.
If flush_interval is specified for a write test, the request queue is drained and a flush is issued
before new writes are made whenever the number of remaining requests is a multiple of flush_interval.
If additionally "--no-drain" is specified, a flush is issued without draining the request queue
first.
If "-n" is specified, the native AIO backend is used if possible. On Linux, this option only works if
"-t none" or "-t directsync" is specified as well.
For write tests, by default a buffer filled with zeros is written. This can be overridden with a
pattern byte specified by pattern.
check [--object objectdef] [--image-opts] [-q] [-f fmt] [--output=ofmt] [-r [leaks | all]] [-T src_cache]
[-U] filename
Perform a consistency check on the disk image filename. The command can output in the format ofmt
which is either "human" or "json". The JSON output is an object of QAPI type "ImageCheck".
If "-r" is specified, qemu-img tries to repair any inconsistencies found during the check. "-r leaks"
repairs only cluster leaks, whereas "-r all" fixes all kinds of errors, with a higher risk of
choosing the wrong fix or hiding corruption that has already occurred.
Only the formats "qcow2", "qed" and "vdi" support consistency checks.
In case the image does not have any inconsistencies, check exits with 0. Other exit codes indicate
the kind of inconsistency found or if another error occurred. The following table summarizes all exit
codes of the check subcommand:
0 Check completed, the image is (now) consistent
1 Check not completed because of internal errors
2 Check completed, image is corrupted
3 Check completed, image has leaked clusters, but is not corrupted
63 Checks are not supported by the image format
If "-r" is specified, exit codes representing the image state refer to the state after (the attempt
at) repairing it. That is, a successful "-r all" will yield the exit code 0, independently of the
image state before.
commit [--object objectdef] [--image-opts] [-q] [-f fmt] [-t cache] [-b base] [-d] [-p] filename
Commit the changes recorded in filename in its base image or backing file. If the backing file is
smaller than the snapshot, then the backing file will be resized to be the same size as the snapshot.
If the snapshot is smaller than the backing file, the backing file will not be truncated. If you
want the backing file to match the size of the smaller snapshot, you can safely truncate it yourself
once the commit operation successfully completes.
The image filename is emptied after the operation has succeeded. If you do not need filename
afterwards and intend to drop it, you may skip emptying filename by specifying the "-d" flag.
If the backing chain of the given image file filename has more than one layer, the backing file into
which the changes will be committed may be specified as base (which has to be part of filename's
backing chain). If base is not specified, the immediate backing file of the top image (which is
filename) will be used. Note that after a commit operation all images between base and the top image
will be invalid and may return garbage data when read. For this reason, "-b" implies "-d" (so that
the top image stays valid).
compare [--object objectdef] [--image-opts] [-f fmt] [-F fmt] [-T src_cache] [-p] [-q] [-s] [-U]
filename1 filename2
Check if two images have the same content. You can compare images with different format or settings.
The format is probed unless you specify it by -f (used for filename1) and/or -F (used for filename2)
option.
By default, images with different size are considered identical if the larger image contains only
unallocated and/or zeroed sectors in the area after the end of the other image. In addition, if any
sector is not allocated in one image and contains only zero bytes in the second one, it is evaluated
as equal. You can use Strict mode by specifying the -s option. When compare runs in Strict mode, it
fails in case image size differs or a sector is allocated in one image and is not allocated in the
second one.
By default, compare prints out a result message. This message displays information that both images
are same or the position of the first different byte. In addition, result message can report
different image size in case Strict mode is used.
Compare exits with 0 in case the images are equal and with 1 in case the images differ. Other exit
codes mean an error occurred during execution and standard error output should contain an error
message. The following table sumarizes all exit codes of the compare subcommand:
0 Images are identical
1 Images differ
2 Error on opening an image
3 Error on checking a sector allocation
4 Error on reading data
convert [--object objectdef] [--image-opts] [--target-image-opts] [-U] [-C] [-c] [-p] [-q] [-n] [-f fmt]
[-t cache] [-T src_cache] [-O output_fmt] [-B backing_file] [-o options] [-l snapshot_param] [-S
sparse_size] [-m num_coroutines] [-W] filename [filename2 [...]] output_filename
Convert the disk image filename or a snapshot snapshot_param to disk image output_filename using
format output_fmt. It can be optionally compressed ("-c" option) or use any format specific options
like encryption ("-o" option).
Only the formats "qcow" and "qcow2" support compression. The compression is read-only. It means that
if a compressed sector is rewritten, then it is rewritten as uncompressed data.
Image conversion is also useful to get smaller image when using a growable format such as "qcow": the
empty sectors are detected and suppressed from the destination image.
sparse_size indicates the consecutive number of bytes (defaults to 4k) that must contain only zeros
for qemu-img to create a sparse image during conversion. If sparse_size is 0, the source will not be
scanned for unallocated or zero sectors, and the destination image will always be fully allocated.
You can use the backing_file option to force the output image to be created as a copy on write image
of the specified base image; the backing_file should have the same content as the input's base image,
however the path, image format, etc may differ.
If a relative path name is given, the backing file is looked up relative to the directory containing
output_filename.
If the "-n" option is specified, the target volume creation will be skipped. This is useful for
formats such as "rbd" if the target volume has already been created with site specific options that
cannot be supplied through qemu-img.
Out of order writes can be enabled with "-W" to improve performance. This is only recommended for
preallocated devices like host devices or other raw block devices. Out of order write does not work
in combination with creating compressed images.
num_coroutines specifies how many coroutines work in parallel during the convert process (defaults to
8).
create [--object objectdef] [-q] [-f fmt] [-b backing_file] [-F backing_fmt] [-u] [-o options] filename
[size]
Create the new disk image filename of size size and format fmt. Depending on the file format, you can
add one or more options that enable additional features of this format.
If the option backing_file is specified, then the image will record only the differences from
backing_file. No size needs to be specified in this case. backing_file will never be modified unless
you use the "commit" monitor command (or qemu-img commit).
If a relative path name is given, the backing file is looked up relative to the directory containing
filename.
Note that a given backing file will be opened to check that it is valid. Use the "-u" option to
enable unsafe backing file mode, which means that the image will be created even if the associated
backing file cannot be opened. A matching backing file must be created or additional options be used
to make the backing file specification valid when you want to use an image created this way.
The size can also be specified using the size option with "-o", it doesn't need to be specified
separately in this case.
dd [--image-opts] [-U] [-f fmt] [-O output_fmt] [bs=block_size] [count=blocks] [skip=blocks] if=input
of=output
Dd copies from input file to output file converting it from fmt format to output_fmt format.
The data is by default read and written using blocks of 512 bytes but can be modified by specifying
block_size. If count=blocks is specified dd will stop reading input after reading blocks input
blocks.
The size syntax is similar to dd(1)'s size syntax.
info [--object objectdef] [--image-opts] [-f fmt] [--output=ofmt] [--backing-chain] [-U] filename
Give information about the disk image filename. Use it in particular to know the size reserved on
disk which can be different from the displayed size. If VM snapshots are stored in the disk image,
they are displayed too.
If a disk image has a backing file chain, information about each disk image in the chain can be
recursively enumerated by using the option "--backing-chain".
For instance, if you have an image chain like:
base.qcow2 <- snap1.qcow2 <- snap2.qcow2
To enumerate information about each disk image in the above chain, starting from top to base, do:
qemu-img info --backing-chain snap2.qcow2
The command can output in the format ofmt which is either "human" or "json". The JSON output is an
object of QAPI type "ImageInfo"; with "--backing-chain", it is an array of "ImageInfo" objects.
"--output=human" reports the following information (for every image in the chain):
image
The image file name
file format
The image format
virtual size
The size of the guest disk
disk size
How much space the image file occupies on the host file system (may be shown as 0 if this
information is unavailable, e.g. because there is no file system)
cluster_size
Cluster size of the image format, if applicable
encrypted
Whether the image is encrypted (only present if so)
cleanly shut down
This is shown as "no" if the image is dirty and will have to be auto-repaired the next time it is
opened in qemu.
backing file
The backing file name, if present
backing file format
The format of the backing file, if the image enforces it
Snapshot list
A list of all internal snapshots
Format specific information
Further information whose structure depends on the image format. This section is a textual
representation of the respective "ImageInfoSpecific*" QAPI object (e.g. "ImageInfoSpecificQCow2"
for qcow2 images).
map [--object objectdef] [--image-opts] [-f fmt] [--output=ofmt] [-U] filename
Dump the metadata of image filename and its backing file chain. In particular, this commands dumps
the allocation state of every sector of filename, together with the topmost file that allocates it in
the backing file chain.
Two option formats are possible. The default format ("human") only dumps known-nonzero areas of the
file. Known-zero parts of the file are omitted altogether, and likewise for parts that are not
allocated throughout the chain. qemu-img output will identify a file from where the data can be
read, and the offset in the file. Each line will include four fields, the first three of which are
hexadecimal numbers. For example the first line of:
Offset Length Mapped to File
0 0x20000 0x50000 /tmp/overlay.qcow2
0x100000 0x10000 0x95380000 /tmp/backing.qcow2
means that 0x20000 (131072) bytes starting at offset 0 in the image are available in
/tmp/overlay.qcow2 (opened in "raw" format) starting at offset 0x50000 (327680). Data that is
compressed, encrypted, or otherwise not available in raw format will cause an error if "human" format
is in use. Note that file names can include newlines, thus it is not safe to parse this output
format in scripts.
The alternative format "json" will return an array of dictionaries in JSON format. It will include
similar information in the "start", "length", "offset" fields; it will also include other more
specific information:
- whether the sectors contain actual data or not (boolean field "data"; if false, the sectors are
either unallocated or stored as optimized all-zero clusters);
- whether the data is known to read as zero (boolean field "zero");
- in order to make the output shorter, the target file is expressed as a "depth"; for example, a
depth of 2 refers to the backing file of the backing file of filename.
In JSON format, the "offset" field is optional; it is absent in cases where "human" format would omit
the entry or exit with an error. If "data" is false and the "offset" field is present, the
corresponding sectors in the file are not yet in use, but they are preallocated.
For more information, consult include/block/block.h in QEMU's source code.
measure [--output=ofmt] [-O output_fmt] [-o options] [--size N | [--object objectdef] [--image-opts] [-f
fmt] [-l snapshot_param] filename]
Calculate the file size required for a new image. This information can be used to size logical
volumes or SAN LUNs appropriately for the image that will be placed in them. The values reported are
guaranteed to be large enough to fit the image. The command can output in the format ofmt which is
either "human" or "json". The JSON output is an object of QAPI type "BlockMeasureInfo".
If the size N is given then act as if creating a new empty image file using qemu-img create. If
filename is given then act as if converting an existing image file using qemu-img convert. The
format of the new file is given by output_fmt while the format of an existing file is given by fmt.
A snapshot in an existing image can be specified using snapshot_param.
The following fields are reported:
required size: 524288
fully allocated size: 1074069504
The "required size" is the file size of the new image. It may be smaller than the virtual disk size
if the image format supports compact representation.
The "fully allocated size" is the file size of the new image once data has been written to all
sectors. This is the maximum size that the image file can occupy with the exception of internal
snapshots, dirty bitmaps, vmstate data, and other advanced image format features.
snapshot [--object objectdef] [--image-opts] [-U] [-q] [-l | -a snapshot | -c snapshot | -d snapshot]
filename
List, apply, create or delete snapshots in image filename.
rebase [--object objectdef] [--image-opts] [-U] [-q] [-f fmt] [-t cache] [-T src_cache] [-p] [-u] -b
backing_file [-F backing_fmt] filename
Changes the backing file of an image. Only the formats "qcow2" and "qed" support changing the backing
file.
The backing file is changed to backing_file and (if the image format of filename supports this) the
backing file format is changed to backing_fmt. If backing_file is specified as "" (the empty string),
then the image is rebased onto no backing file (i.e. it will exist independently of any backing
file).
If a relative path name is given, the backing file is looked up relative to the directory containing
filename.
cache specifies the cache mode to be used for filename, whereas src_cache specifies the cache mode
for reading backing files.
There are two different modes in which "rebase" can operate:
Safe mode
This is the default mode and performs a real rebase operation. The new backing file may differ
from the old one and qemu-img rebase will take care of keeping the guest-visible content of
filename unchanged.
In order to achieve this, any clusters that differ between backing_file and the old backing file
of filename are merged into filename before actually changing the backing file.
Note that the safe mode is an expensive operation, comparable to converting an image. It only
works if the old backing file still exists.
Unsafe mode
qemu-img uses the unsafe mode if "-u" is specified. In this mode, only the backing file name and
format of filename is changed without any checks on the file contents. The user must take care of
specifying the correct new backing file, or the guest-visible content of the image will be
corrupted.
This mode is useful for renaming or moving the backing file to somewhere else. It can be used
without an accessible old backing file, i.e. you can use it to fix an image whose backing file
has already been moved/renamed.
You can use "rebase" to perform a "diff" operation on two disk images. This can be useful when you
have copied or cloned a guest, and you want to get back to a thin image on top of a template or base
image.
Say that "base.img" has been cloned as "modified.img" by copying it, and that the "modified.img"
guest has run so there are now some changes compared to "base.img". To construct a thin image called
"diff.qcow2" that contains just the differences, do:
qemu-img create -f qcow2 -b modified.img diff.qcow2
qemu-img rebase -b base.img diff.qcow2
At this point, "modified.img" can be discarded, since "base.img + diff.qcow2" contains the same
information.
resize [--object objectdef] [--image-opts] [-f fmt] [--preallocation=prealloc] [-q] [--shrink] filename
[+ | -]size
Change the disk image as if it had been created with size.
Before using this command to shrink a disk image, you MUST use file system and partitioning tools
inside the VM to reduce allocated file systems and partition sizes accordingly. Failure to do so
will result in data loss!
When shrinking images, the "--shrink" option must be given. This informs qemu-img that the user
acknowledges all loss of data beyond the truncated image's end.
After using this command to grow a disk image, you must use file system and partitioning tools inside
the VM to actually begin using the new space on the device.
When growing an image, the "--preallocation" option may be used to specify how the additional image
area should be allocated on the host. See the format description in the "NOTES" section which values
are allowed. Using this option may result in slightly more data being allocated than necessary.
NOTES
Supported image file formats:
raw Raw disk image format (default). This format has the advantage of being simple and easily exportable
to all other emulators. If your file system supports holes (for example in ext2 or ext3 on Linux or
NTFS on Windows), then only the written sectors will reserve space. Use "qemu-img info" to know the
real size used by the image or "ls -ls" on Unix/Linux.
Supported options:
"preallocation"
Preallocation mode (allowed values: "off", "falloc", "full"). "falloc" mode preallocates space
for image by calling posix_fallocate(). "full" mode preallocates space for image by writing data
to underlying storage. This data may or may not be zero, depending on the storage location.
qcow2
QEMU image format, the most versatile format. Use it to have smaller images (useful if your
filesystem does not supports holes, for example on Windows), optional AES encryption, zlib based
compression and support of multiple VM snapshots.
Supported options:
"compat"
Determines the qcow2 version to use. "compat=0.10" uses the traditional image format that can be
read by any QEMU since 0.10. "compat=1.1" enables image format extensions that only QEMU 1.1 and
newer understand (this is the default). Amongst others, this includes zero clusters, which allow
efficient copy-on-read for sparse images.
"backing_file"
File name of a base image (see create subcommand)
"backing_fmt"
Image format of the base image
"encryption"
If this option is set to "on", the image is encrypted with 128-bit AES-CBC.
The use of encryption in qcow and qcow2 images is considered to be flawed by modern cryptography
standards, suffering from a number of design problems:
- The AES-CBC cipher is used with predictable initialization vectors based on the sector
number. This makes it vulnerable to chosen plaintext attacks which can reveal the existence
of encrypted data.
- The user passphrase is directly used as the encryption key. A poorly chosen or short
passphrase will compromise the security of the encryption.
- In the event of the passphrase being compromised there is no way to change the passphrase to
protect data in any qcow images. The files must be cloned, using a different encryption
passphrase in the new file. The original file must then be securely erased using a program
like shred, though even this is ineffective with many modern storage technologies.
- Initialization vectors used to encrypt sectors are based on the guest virtual sector number,
instead of the host physical sector. When a disk image has multiple internal snapshots this
means that data in multiple physical sectors is encrypted with the same initialization
vector. With the CBC mode, this opens the possibility of watermarking attacks if the attack
can collect multiple sectors encrypted with the same IV and some predictable data. Having
multiple qcow2 images with the same passphrase also exposes this weakness since the
passphrase is directly used as the key.
Use of qcow / qcow2 encryption is thus strongly discouraged. Users are recommended to use an
alternative encryption technology such as the Linux dm-crypt / LUKS system.
"cluster_size"
Changes the qcow2 cluster size (must be between 512 and 2M). Smaller cluster sizes can improve
the image file size whereas larger cluster sizes generally provide better performance.
"preallocation"
Preallocation mode (allowed values: "off", "metadata", "falloc", "full"). An image with
preallocated metadata is initially larger but can improve performance when the image needs to
grow. "falloc" and "full" preallocations are like the same options of "raw" format, but sets up
metadata also.
"lazy_refcounts"
If this option is set to "on", reference count updates are postponed with the goal of avoiding
metadata I/O and improving performance. This is particularly interesting with cache=writethrough
which doesn't batch metadata updates. The tradeoff is that after a host crash, the reference
count tables must be rebuilt, i.e. on the next open an (automatic) "qemu-img check -r all" is
required, which may take some time.
This option can only be enabled if "compat=1.1" is specified.
"nocow"
If this option is set to "on", it will turn off COW of the file. It's only valid on btrfs, no
effect on other file systems.
Btrfs has low performance when hosting a VM image file, even more when the guest on the VM also
using btrfs as file system. Turning off COW is a way to mitigate this bad performance. Generally
there are two ways to turn off COW on btrfs: a) Disable it by mounting with nodatacow, then all
newly created files will be NOCOW. b) For an empty file, add the NOCOW file attribute. That's
what this option does.
Note: this option is only valid to new or empty files. If there is an existing file which is COW
and has data blocks already, it couldn't be changed to NOCOW by setting "nocow=on". One can issue
"lsattr filename" to check if the NOCOW flag is set or not (Capital 'C' is NOCOW flag).
Other
QEMU also supports various other image file formats for compatibility with older QEMU versions or
other hypervisors, including VMDK, VDI, VHD (vpc), VHDX, qcow1 and QED. For a full list of supported
formats see "qemu-img --help". For a more detailed description of these formats, see the QEMU
Emulation User Documentation.
The main purpose of the block drivers for these formats is image conversion. For running VMs, it is
recommended to convert the disk images to either raw or qcow2 in order to achieve good performance.
SEE ALSO
The HTML documentation of QEMU for more precise information and Linux user mode emulator invocation.
AUTHOR
Fabrice Bellard
2024-10-20 QEMU-IMG.1(1)